300 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



table, and there is therefore an intermediate belt between the root 

 zone, or belt of soil moisture, and the zone of saturation. 



Replenishment of the underground reservoirs. — The underground 

 reservoirs are replenished, or recharged, with water from atmospheric 

 sources. Nearly all hydrologists believe that the recharge is essentially 

 all from rain and snow or from streams fed by rain and snow, but 

 there are still a few hydrologists who believe that subsurface con- 

 densation is a substantial source. The amount of recharge from a 

 given amoimt and land of precipitation varies with the absorptive or 

 intake capacity of the soil or other surficial material and inversely 

 with the capacity of the soil to hold the water for plant use instead 

 of allowing it to percolate downward to the water table. 



The intake capacity constitutes a large subject with many ramifi- 

 cations. It includes questions as to the efi'ects of the vegetable mold 

 in the forested areas and of the natural sod, and, on the other hand, 

 the effects of grazing and of the cultivation of the soil. It includes 

 also the problems of artificial recharge by spreading stream water or 

 by other means, and of the silting up of the natural recharge channels 

 by surface storage or other manipulation. The surface conditions of 

 both forests and sod-covered prairies are favorable to intake by keep- 

 ing the rain and snow water clean and thus permitting it to percolate 

 downward through the available ducts and pores without clogging 

 them, whereas under some conditions the cultivation of the soil tends 

 to decrease the intake capacity, especially in heavy and prolonged 

 rains, by puddling the top layer of soil and choking the intake open- 

 ings. However, forests consume large quantities of water by transpira- 

 tion, which tends to offset their large intake. The conflicting results 

 obtained by different investigatoi's as to the effects of forests on the 

 water table and on the flow of springs and streams, as compared to 

 the effects of cleared or cultivated land, are in part due to the fact that 

 in some places and at some times the balance is actually on one side 

 and in others on the other side. Relatively little investigation has as 

 yet been made of transpiration on the sod-covered prairies and of the 

 effects of brealdng up and cultivating the prairie lands. It appears 

 probable that there is a basis in fact for the prevalent belief that the 

 advent of the white men in this country was attended by a certain 

 amount of lowering of ground water levels and of decrease in the flow 

 of springs and streams. 



It appears that artificial recharge by water spreading, by impound- 

 ing of surface water and regulation of stream flow, and perhaps by 

 drainage Into wells, has large possibilities for increasing the perennial 

 supply of ground water in certain specific areas of heavy consumption 

 in which the natural conditions are favorable. On the other hand, for 

 the country as a whole, recharge by such means will remain small in 

 comparison to the total natural recharge and the total discharge of 



